Cleaner technologies for asphalt mixtures combining reuse of residual aggregates, waste crumb rubber and warm mix asphalt additive

The reuse of waste materials and residual aggregates as well as the reduction of emissions has become vitally important for the environment, the economy and logistics of the asphalt paving industry. This study characterizes the performance and production properties of asphalt mixtures with waste crumb-rubber modifier from scrap tires, a warm mix asphalt surfactant additive and residual poor-quality volcanic aggregates as the single mineral component. The combination of these three cleaner technologies provides a promising solution to produce more sustainable materials by reusing two different types of waste and decreasing the manufacturing temperature at the same time. The compactability, stiffness modulus and fatigue performance characteristics were assessed in the laboratory for different low production temperatures and compared to conventional mixtures. The results indicate that these rubberized warm asphalt mixtures with residual vesicular and scoriaceous aggregates comply with the technical specifications for paving materials. The dynamic properties are maintained or even improved while reusing waste materials and allowing reductions of the manufacturing and compaction temperatures up to 20 °C, therefore, decreasing energy consumption and emissions.


a) b )
Supplementary Figure S4. Images from scanning electron microscopy [magnification x200]: a) Fatigue microcrack at the interfacial contact between the volcanic aggregate particle of type vesicular basalt and the crumb-rubber modified binder (CRMB); b) Fatigue micro-crack in the CRMB.

Compactability test
Standard: EN 12697-10 Procedure: Method of same sample for all energy levels on cylindrical specimens compacted by impact up to 2 x 100 blows (100 blows per each side) using the Marshall compactor (EN 12697-30), monitoring and recording the change of the specimen thickness during the compaction process.
Equation (1) relates the thickness variation of the specimen, the compaction energy applied and the resistance to compaction: where: t(E) = thickness of the compacted specimen as a function of the compaction energy (mm); t∞ = minimum possible thickness of the specimen (mm); t 0 = initial thickness of the specimen (mm); E = compaction energy by impact, expressed considering 21 Nm as the unit (number of blows); T = resistance to compaction by impact (number of blows).

Procedure:
By indirect tensile test on cylindrical specimens [IT-CY] compacted by impact with 2 x 75 blows, k = 0.6, T = 20 ºC, f = 2.2 Hz) and the stiffness modulus obtained according to the equation (2).
In each IT-CY test five haversine repeated loading pulses with the corresponding intermediate rest periods were applied, controlling the loading time during tests. The ratio of rest periods to loading time (R/D) were between 20.5 and 22.2 (R/D should be equal or greater to 9 to achieve an acceptable range of error in measurement of resilient modulus). The load surface factor (k, related to the shape of the loading pulse curve or waveform), rise time (from zero load up to peak load), strain and stiffness modulus in each loading pulse were also obtained.
where: S m = Stiffness modulus measured by the test (MPa); F = maximum vertical load applied (N); z = horizontal strain amplitude during the loading cycle (mm); h = average specimen thickness (mm); μ = Poisson ratio (a constant value of 0.35 was assumed). According to EN 12697-26, it was verified that the rise time was between 120 and 128 ms, the load surface factor (k) between 0.5 and 0.8, and the strain between 3 and 20 µm. The stiffness modulus measured was corrected by Equation (3) where: S' m = Stiffness modulus corrected for a load surface factor of 0.6 (MPa); k = load surface factor measured.
Moreover, as this is a non-destructive test, each specimen was tested at two different diametrically opposite positions.
The load amplitude was 2700 N, the pulse frequency 2.2 Hz and the resting time 2750 ms and the temperature during the test was 20 ºC. Afterwards, averages of these moduli were calculated as well as the mean values for each set of the three specimens with the same bitumen content.
In each 4PB-PR test a repeated haversine load was applied on prismatic specimens. The four supporting points enable a flexural load with a constant strain within the two intermediate clamps of the beam specimen. The prismatic beams, obtained by cutting slab specimens, were tested at 20 ºC. According to EN 12697-24, the initial stiffness modulus calculated for each mixture specimen (S mix ) is obtained from the load, displacement and phase angle after 100 load applications. The test with strain control continues until the modulus diminishes up to half its initial value or up to -5 -specimen failure. The fatigue law for fatigue life prediction was obtained by linear regression of the logarithms of the number of cycles and the logarithms of the initial strain amplitude, according to: where: ε 0 = initial strain amplitude; N = load cycles until fatigue failure; a 0 , a 1 = parameters (material constants) obtained from the 4PB-PR fatigue test.